This invention relates to dynamoelectric machines and more particularly to homopolar generators having means for reducing circulating currents in their current collecting zones.
Homopolar generators have been successfully designed for providing short duration pulses having a peak current level in excess of a million amperes DC. Such generators usually include a cylindrical rotor of either a drum or disc configuration, rotatably mounted within a stator structure. A field coil encircling the rotor and connected to an external current supply provides an applied magnetic field excitation passing through the rotor. The applied magnetic field excitation is usually confined and directed by a ferromagnetic stator structure surrounding the field coil and all, or a portion of, the rotor. When the rotor is spinning, free electrons within the rotor experience an electromotive force resulting from their interaction with the applied magnetic field excitation. Current collecting brushes are lowered onto the spinning rotor to allow an electrical current to flow under the influence of this electromotive force through return conductors to an external circuit and then back onto the rotor through additional brushes at a different location. It has been found that extremely high current pulses may be obtained by using a relatively low power conventional prime mover to store initial energy in the rotor by gradually motoring the rotor up to the desired rotational speed.
In order to achieve weight and volume reductions, the magnetic rotor of a homopolar generator may be replaced by a hollow non-magnetic cylinder supported on a shaft by spokes to form an air core machine. At the same time, the magnetic stator may be replaced by a non-magnetic stator structure. Since such an air core homopolar generator would have no preferred magnetic paths, magnetic flux crossing the rotor surface can possess significant axial as well as radial components. Circulating currents caused by relatively high radial magnetic flux components in the current collection zone of homopolar generators are responsible for many of the limitations of current machines. Minimization of such currents by reduction of the radial components of the magnetic field in the current collection region, while maintaining high radial fields in the active region, is desirable.